Fuel cell with novel reaction layer

ABSTRACT

A fuel cell includes a reaction layer composed of a catalyst carrier, formed of a compound having inorganic electron conductor units and inorganic proton conductor units in its molecular structure, and a catalyst supported on the catalyst carrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

Related subject matter is disclosed and claimed in U.S. application Ser.No. 10/509,752 filed Sep. 30, 2004 and in U.S. application Ser. No.10/667,974, filed Sep. 23, 2003.

FIELD OF THE INVENTION

The present invention relates to fuel cells. More particularly, thepresent invention relates to improvement of the reaction layer of a fuelcell.

BACKGROUND OF THE INVENTION

The reaction layer of a fuel cell is formed between a proton exchangemembrane and a backing layer and supports a catalyst for accelerating anelectrochemical reaction. In the reaction layer, on an air cathode sidefor example, protons passing through the proton exchange membrane andelectrons transferred to the air cathode are conducted to the catalyst,for reaction between the oxygen and protons diffused onto the catalyst.Thus, the reaction layer should exhibit both proton conduction andelectron conduction in order to minimize the transfer loss of oxygen,protons and electrons. To this end, a mixture of poly electrolyteshaving catalysts supported on their surfaces, e.g. carbon particles(exhibiting electron conduction) and Nafion (trade name, manufactured byE.I du Pont de Nemours) which exhibits ionic conduction have been usedin a fuel cell (see FIG. 1B).

However, if a material having ionic conduction and a material havingelectronic conduction are used together, it is difficult to mix them soas to achieve complete uniformity. As a result, protons and electronscannot be uniformly transferred to all catalyst particles.

To solve the foregoing problem, there have been proposed a variety ofmixed conductors for use as carriers designed for supporting catalysts,which carriers exhibit both ionic conduction and electron conductionusing one material (A carrier designed for supporting a catalyst will behereinafter referred to as a “catalyst-supporting carrier”).

Organic catalyst-supporting carriers are disclosed in JP2001-202971A,JP2001-110428A, JP2003-68321A and JP2002-536787A. However, since theorganic catalyst-supporting carriers are made of organic materials, theypresent many problems in terms of durability and heat resistance whichare obstacles to practical use.

In addition, inorganic catalyst-supporting carriers which conductelectrons and oxygen ions are disclosed in JP1998-255832A,JP1999-335165A, JP2000-251533A, and JP2000-18811A. However, theinorganic catalyst-supporting carriers which transfer electrons andoxygen ions have high operating temperatures (about 800° C.). Due tosuch high operating temperatures, these inorganic catalyst-supportingcarriers are inappropriate for use in small-sized fuel cells, forexample in vehicles and cellular phones.

Prior to the present invention, no catalyst-supporting carrierexhibiting both proton conduction and electron conduction had beendeveloped which operates within a moderate temperature range (roomtemperature to 200° C.) and which can be used in fuel cells.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to solve theforegoing problems by providing a novel inorganic catalyst-supportingcarrier, a supported metal catalyst wherein the support is the novelcarrier and a fuel cell reaction layer employing the supported metalcatalyst, as well as a fuel cell that incorporates such a reactionlayer.

Thus, the present invention provides a fuel cell comprising a reactionlayer substantially composed of a catalyst-supporting carrier, in theform of a compound of an inorganic electron conductor and an inorganicproton conductor, bound together in an inorganic polymeric molecularstructure, and a catalyst supported thereon.

Being a single compound, i.e. having a single molecular structure, thiscatalyst-supporting carrier (or “mixed conductor”) as a whole providesboth electron conduction and proton conduction while the electronconductors and the proton conductors thereof are strongly bondedtogether in a single molecular structure which is substantiallyinsoluble in water.

Preferably, the electron conductor is obtained by carbonizing an organicmaterial.

The supported metal catalyst includes a carrier and, supported thereon,a metal catalyst, such as platinum or a platinum alloy, suitable for afuel cell reaction. The term “supported metal catalyst” as used hereinrefers to any suitable catalyst supported on the above-describedcatalyst-supporting carrier. In a supported metal catalyst (platinum orother catalyst suitable for a fuel cell reaction) of the presentinvention, as shown in FIG. 1A, electrons, protons, and oxygen aresupplied to all the supported catalyst particles. This permits fullutilization of all the catalyst. If such a supported metal catalyst isused to form a reaction layer for a fuel cell, the efficiency of thecatalyst and thus that of the fuel cell are improved.

In contrast, in a conventional fuel cell reaction layer of a typecurrently in use that employs both (1) a supported metal catalystcomposed of an electron conductive carrier (such as carbon particles)that supports a catalyst and (2) an ion exchange resin (such as Nafion),as shown in FIG. 1B, the ion exchange resin may be prevented from cominginto contact with the catalyst supported in the recesses in the electronconductive carrier. Since no proton is supplied to the catalyst in suchrecesses, it does not contribute to the fuel cell reaction.

Gas travels in an ion exchange resin at a speed far slower than in gasphase. Accordingly, an excessive coating of an ion exchange resin blocksgas supply and thus adversely affects the performance of the fuel cell.Furthermore, ion exchange resins experience dimensional change during adrying-wetting cycle. This change in dimensions may cause some catalyststo separate from the carrier.

The inorganic electron conductor may be of the type having a main chaincontaining one of or both of carbon-carbon double bonds andcarbon-carbon triple bonds, the main chain contributing to the electronconduction function, as shown in FIGS. 2 and 3. Alternatively, a typewhich transfers electrons through a side chain may be used.

The preferred inorganic electron conductor is that obtained bycarbonizing an organic compound having a π bond. Examples of the organiccompounds having a π bond include aliphatic hydrocarbons, aromatichydrocarbons and derivatives thereof. At least one such compound is usedas the organic compound to be carbonized. Typical examples of π bondorganic compounds include polyacetylene, resorcinol, phenol,phenylphenol, polyaniline, polypyrrole, polythiophene, phenylphosphonicacid, phenylsilane alkoxide, pyrogallol, and dihydroxybiphenyl.

Further, the inorganic material used for the electron conductor can be acarbonaceous material such as graphite or a carbon nanotube or ametallic material containing a metal such as gold, palladium, platinum,magnesium, lithium or titanium, or an alloy thereof.

The inorganic proton conductor may be a phosphorus-containing compound,a sulfur-containing compound, carbonic acid, boric acid, or inorganicsolid-state acid, particularly at least one aphosphorus-containing acid,such as phosphoric acid, or a phosphoric acid ester, sulfuric acid, asulfuric acid ester, tungsten oxide hydroxide, rhenium oxide hydroxide,silicon oxide, tin oxide, zirconia oxide, tungstophosphoric acid, andtungstosilicic acid.

According to the present invention, inorganic electron-conducting unitsand inorganic proton-conducting units may together form the molecularstructure of a single compound (inorganic polymer) in which the electronconductor and the proton conductor are strongly bonded together. As aresult, when the catalyst-supporting carrier of the present invention isimmersed in water, hardly any proton conductors are desorbed.Additionally, the mixed conductor, as a whole, provides both electronconduction and proton conduction.

The foregoing compound maybe formed (i.e., the electron conductors maybe fixed to the proton conductors) by covalent bonding in an inorganic,polymeric molecular structure. Alternatively, the bonding may be byintercalation or inclusion. However, depending on production processconditions, these different forms may possibly be mixed.

The type of bonding, chemical (covalent bonding) or physical(intercalation or inclusion) is determined by the types of the materialsserving as the electron conductor and the proton conductor. For example,if the electron conductor is an inorganic material obtained bycarbonizing an organic material, the bonding may be mainly covalentbonding. If the electron conductor is a metallic material and the protonconductor is an inorganic material, such as an oxide, for example, thetwo different conductors can be fixed to each other by covalent bondingor by inclusion.

Covalent bonding of an electron conductor and a proton conductor in acrystalline inorganic, polymeric molecular structure is illustrated inFIGS. 2, 3, 4 and 5. Since the covalently bonded electron conductors 1or 3 and the proton conductors 2 are in close proximity, both theelectron conductors and the proton conductors contact the catalystparticles supported thereon (e.g., platinum) at the nano level as shownin FIG. 1A. Accordingly, it is possible to supply electrons and protonsnecessary for a catalytic reaction to the catalyst in proper quantities.

The catalyst-supporting carrier is formed as follows.

First, a high molecular weight precursor is formed by polymerizing anorganic compound having a π bond together with a proton conductingmaterial. In this high molecular weight polymer precursor, the carbonatoms of the organic compound are believed to form electron conductingmain chains having a π bond and covalent bonds with the protonconductors which form bridges between the carbon units of the carbonmain chain of the electron conductor. By incorporating the protonconductor into the main chain of the inorganic polymer in sufficientquantities, the proton conductors are sufficiently close to provideproton conduction therebetween.

As shown in FIGS. 2, 3, 4 and 5 the mixed conductor carrier of thepresent invention may be in the form of an inorganic polymer wherein themolecular structure of the polymer includes both electron conductingportions (groups or units) and proton conducting portions (groups orunits).

Alternatively, a precursor may be obtained by dispersing a protonconductor in a polymer of an organic compound having a π bond.

In a case where degree of polymerization is low, the result isdispersion of the proton conductor in the polymerized organic compound.Where the degree of polymerization is not sufficient, the result is aprecursor containing both proton conductor covalent bonding to anorganic compound forming an electronic conductor and dispersed protonconductors, isolated from the covalently bonded proton conductor.

When one of the precursors described above is pyrolyzed in an inertatmosphere, the organic compound is carbonized and thereby convertedinto an inorganic material providing electron conduction.

Because the proton conductor is stably fixed within electron conductingcarbon skeletons, proton conduction is ensured. It is considered thatthe proton conduction is attained by arrangement of the protonconductors in close proximity to each other. As shown in FIGS. 1A, 1Band 2, where the proton conductors bridge the carbon groups (or“units”), the positions of the proton conductors are fixed, therebyensuring the proton conduction by the interaction between the protonconductors.

If the proton conductors are released from the carbon skeletons or ifthe proton conductors are not bound to the carbon skeletons inconversion of the precursor, it is believed that the proton conductorsbecome intercalated into the carbon main chain or included within themesh structure formed by the carbon main chain. In any case, it isbelieved that proton conduction can be ensured as long as the protonconductors are in close proximity.

As can be seen, since the proton conductors are integrated into acompound and are bonded, intercalated or included between the carbonskeletons, the proton conductors are fixed within the catalyst support(mixed conductor) (do not “float”). Thus, even if the mixed conductor isused within an environment where water is present, the proton conductoris not dissolved out by the water. The mixed conductor as a wholeprovides both electron conduction and proton conduction and the protonconduction is not significantly reduced by exposure to water.

Preferably, the precursor is heated or pressure-heated prior topyrolyzing. Such heating or pressure-heating of the precursor willresult in an increased phosphorus content after pyrolyzation. The methodemployed for heating or pressure-heating the precursor is notspecifically limited; rather, any commonly employed method may beutilized.

The precursor is heated to a boil and the steam generated is condensedby cooling and returned to the reaction vessel. Refluxing is provided bya cooled condenser preferably installed in association with the reactionvessel. Such refluxing permits the precursor to be heated at atmosphericpressure. The specific heating temperature and heating time are selectedto suit the characteristics of the precursor.

The method employed for pressure-heating the precursor is notspecifically limited; however, use of an autoclave is preferred from thestandpoint of workability and other related factors. The pressureapplied to the precursor during heating may be atmospheric pressure oran elevated pressure to suit the characteristics of the precursor.

Examples of the organic compound having a π bond include unsaturatedaliphatic hydrocarbons and aromatic hydrocarbons. More specifically, atleast one of polyacetylene, resorcinol, phenol, phenylphenol,polyaniline, polypyrrole, polythiophene, phenylphosphonic acid,phenylsilane alkoxide, pyrogallol, and dihydroxybiphenyl can be selectedas the organic compound having a π bond.

Suitable proton conducting materials include phosphorus-containingcompounds, sulfur-containing compounds, carbonic acid, boric acid, andinorganic solid-state acids. Examples of the phosphorus-containingcompounds include phosphoric acid and examples of the sulfur-containingcompounds include sulfuric acid and sulfonic acid. Further, an inorganicproton conducting material can be produced using a derivative of one ofthese compounds as a starting material. Preferably, the protonconducting material is at least one of a phosphorus-containing compound,phosphoric acid, phosphate ester, sulfuric acid, sulfate ester, sulfuricacid, tungsten oxide hydroxide, rhenium oxide hydroxide, silicon oxide,tin oxide, zirconia oxide, tungstophosphoric acid, and tungstosilicicacid.

To convert the organic compound in the precursor into organic units, itis preferable that the precursor is pyrolyzed in an inert atmosphere.

The inert atmosphere can be argon gas, nitrogen gas, helium gas or avacuum. When the precursor is pyrolyzed in such an inert atmosphere, theorganic component of the precursor is carbonized and thereby convertedinto an inorganic material. If the main chain of the organic componenthas a π bond, high electron conduction is ensured. Heating temperatureand heating time are appropriately selected according to thecharacteristics of the precursor.

Simultaneously with or after heating, high energy in a form other thanheat, such as plasma radiation, microwave radiation or ultrasonicradiation, can be applied to the precursor.

As described above, the catalyst-supporting carrier according to thepresent invention is made of inorganic materials and exhibits both anelectron conducting function and a proton conducting function. Inaddition, even in a low temperature range close to a room temperature,the catalyst-supporting carrier functions properly. Further, even ifexposed to water, the catalyst-supporting carrier retains both itselectron conduction and its proton conduction.

Such a catalyst-supporting carrier is capable of supporting a metal(especially a noble or precious metal) having catalytic activity. Themethod for forming the metal catalyst on the carrier is not specificallylimited, i.e. any known method may be used for the purpose of thepresent invention.

Thus, the mixed conductor is used as a carrier for supporting a metal toprovide a supported metal catalyst, which is in turn used to form areaction layer of a fuel cell unit. For example, the supported metalcatalyst may be dispersed in water, alcohol, or any other appropriatemedium to prepare a paste which is applied onto one side of the backinglayer to form a reaction layer. Such a reaction layer, and the backinglayer to which the reaction layer is applied, may be joined to each sideof a proton exchange membrane (a Nafion membrane in this case) so as toprovide a fuel cell unit or a unit cell constituent of a fuel cell. Itis also possible to form a reaction layer by applying the paste to theexposed surface of the backing layer. PTFE, Nafion, and other suitablematerials may be added as a binder to the paste.

Alternatively, the supported metal catalyst in the form of powder is hotpressed to form a reaction layer conforming to the intended shape of theelectrode. Such reaction layers and a proton exchange membrane arelaminated and hot pressed to form an integrally molded article in whichthe proton exchange membrane is interposed between the reaction layers.A backing layer may then be joined to the outside (exposed) surface ofeach reaction layer to fabricate a fuel cell unit.

Thus the present invention provides a supported metal catalystcomprising (1) a carrier that has a molecular structure formed of aninorganic electron conductor and that has an inorganic proton conductorincorporated therein and (2) a noble metal catalyst supported on thecarrier. The carrier (catalyst support) of the present invention has aninorganic, polymeric molecular structure formed of electron conductorunits with proton conductor units incorporated into chains in themolecular structure of the polymer by covalent bonding and/or by fixingthe proton conductors within the polymeric electron conductor byinclusion and/or intercalation.

The present invention also provides a method for producing a supportedmetal catalyst comprising:

obtaining a high molecular weight precursor by mixing and polymerizingat least one carbon source selected from the group consisting ofaliphatic hydrocarbons, aromatic hydrocarbons and derivatives of thealiphatic hydrocarbons and the aromatic hydrocarbons with a protonconducting material;

pyrolyzing the high molecular weight precursor to form a carrier;

coating the carrier with a noble metal catalyst to form a supportednoble metal catalyst.

The at least one member selected from the group consisting of aliphatichydrocarbons, aromatic hydrocarbons and/or derivatives thereof may firstbe polymerized and then mixed the proton conducting material and thenpyrolyzed. In the method of the present invention, the carbon sourcemixed with the proton conducting material is preferably at least onemember selected from the group consisting of polyacetylene, resorcinol,phenol, phenylphenol, polyaniline, polypyrrole, polythiophene,phenylphosphonic acid, phenylsilane alkoxide, pyrogallol, anddihydroxybiphenyl.

The proton conductor is preferably at least one member selected from thegroup consisting of phosphorus-containing compounds, sulfur-containingcompounds, carboxylic acids, boric acid, and inorganic solid-stateacids.

The heating for pyrolyzing the high molecular weight precursor may be atatmospheric pressure or at an elevated pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a catalyst-supporting carrier of thepresent invention, showing the manner in which the carrier supports acatalyst; FIG. 1B shows the manner in which a conventionalcatalyst-supporting carrier supports a catalyst;

FIG. 2 illustrates the molecular structure of a catalyst-supportingcarrier (inorganic polymer) according to the present invention;

FIG. 3 illustrates another molecular structure of a catalyst-supportingcarrier according to the present invention;

FIG. 4 illustrates formation of the polymeric molecular structure of aprecursor;

FIG. 5 illustrates yet another molecular structure of acatalyst-supporting carrier of an embodiment according to the presentinvention;

FIG. 6 is a schematic view of a holder for checking the protonconducting function of the catalyst-supporting carrier;

FIG. 7 is a graph of the current-voltage characteristics of the holdershown in FIG. 6;

FIG. 8 is a graph showing change of the amount of phosphoric acid withtime in the catalyst-supporting carrier of the present invention,immersed in pure water;

FIG. 9 is a cross-sectional view of a fuel cell with a reaction layer(on the air cathode side) composed of a metal-supporting catalystaccording to one embodiment of the invention;

FIG. 10 is a cross-sectional view of an apparatus for measuring thecharacteristics of the fuel cell shown in FIG. 9;

FIG. 11 is a chart comparing the characteristics of the fuel cells withreaction layers made of supported metal catalysts according to certainembodiments of the invention with those of a fuel cell with a reactionlayer made of a supported metal catalyst without proton conductivity;

FIG. 12 is a schematic view of apparatus used in the method of thepresent invention, including a reflux condenser;

FIG. 13 is a schematic view of an autoclave used in another embodimentof the method of the present invention; and

FIG. 14 shows the relationship between the proton conductivity and theamount of phosphorus in the mixed conductor of several embodiments ofthe present invention; and

FIG. 15 shows the structure of a fuel cell of another embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above-stated advantages of the supported metal catalyst for use in afuel cell according to the present invention are illustrated by thefollowing description of preferred embodiments, with reference to thedrawings.

First, a method for producing a supported metal catalyst in accordancewith the following chemical reaction will be described with reference toFIGS. 3 and 4.

Resorcinol (10 g) and formaldehyde (13 ml) are dissolved in water (40ml), and a solution obtained by hydrolyzing trimethyl phosphate isadded. The resultant solution is dehydrated and condensed with Na₂CO₃ asa catalyst to gelate the solution. This gel is dried at 120° C., therebyobtaining a precursor as shown in FIG. 4.

The thus obtained precursor is subjected to pyrolysis (at 500 to 1000°C.) in a nitrogen atmosphere to obtain a catalyst-supporting carrier.This catalyst-supporting carrier has electronic conductor phases (units)7 of a graphite-like structure and proton conductor phases (units) 9 inthe form of phosphoric acid groups arranged in an alternating alignmentas shown in FIG. 5.

The catalyst-supporting carrier thus obtained is ground, pressure-formedinto a plate, put between current collecting plates and a DC current issupplied to the plate-like mixed conductor. Specific resistance of eachembodiment is obtained from a voltage at that time. Measurement was atroom temperature.

Example 1 Example 2 Example 3 Heat treatment 500° C. 800° C. 1000° C.temperature Specific resistance 138 0.35 0.13 (Ω · cm)

The reason for the high specific resistance at a heating temperature of500° C. is considered to be insufficient carbonization of the organicmaterial.

The heating temperature and heating time are parameters that can beappropriately selected according to the structure and properties of theorganic compound.

The specific resistance of each sample (Examples 1-3) was determined asfollows:

1/specific resistance=conductivity=1/(resistance×geometrical surfacearea of sample/thickness),

in which resistance=applied voltage/response current.

The proton conductivity and the electron conductivity (described below)are calculated in the same manner. To calculate the proton conductivity,a sample 11 is sandwiched between a material, such as Nafion membranes,that conduct protons but not electrons. To calculate the electronconductivity, a sample 11 is sandwiched between a material, such ascopper, that conducts electrons but not protons.

Next, a proton conduction test will be described with reference to FIGS.6 and 7.

As shown in FIG. 6, a backing layer 17 consisting of a carbon cloth andcatalyst layer 15 is attached to each of the samples 11 obtained inExamples 1 to 3. In the figure, the catalyst layer 15 is composed ofcarbon particles supporting a platinum catalyst and is equivalent to thestructure of a reaction layer of a prior-art fuel cell. A Nafionmembrane 13 transmits protons but blocks electrons.

The structure shown in FIG. 6 is put in a container, and nitrogen gas orhydrogen gas at a temperature of 60° C. and a relative humidity of 100%is introduced into the container. Monitoring of the voltage-currentcharacteristic of the thus moisture-exposed structure over time is shownin FIG. 7.

As can be seen from FIG. 7, even if a voltage is supplied between thebacking layers 17 while introducing the nitrogen gas, no current iscarried. On the other hand, if hydrogen gas is introduced into thecontainer, it can be seen that a current flows. This demonstrates thatthe sample 11 has proton conduction.

The proton conductivity of each sample is calculated as follows.

Example 1 Example 2 Example 3 Heat treatment 500° C. 800° C. 1000° C.temperature Proton conductivity 2.6 × 10⁻³ 1.3 × 10⁻³ 7.3 × 10⁻⁴ (S/cm)

Further, as comparative examples, the proton conductivities of samplesformed without addition of trimethyl phosphate were tested in the samemanner and found to be as follows.

Comparative Comparative Comparative Example 1 Example 2 Example 3 Heattreatment 500° C. 800° C. 1000° C. temperature Proton conductivity 1.0 ×10⁻⁶ 1.0 × 10⁻⁶ 1.0 × 10⁻⁶ (S/cm) or less or less or less

Comparison of the samples formed with addition of trimethyl phosphatewith those formed without trimethyl phosphate, confirms protonconduction by phosphorus.

The relationship between immersion time and phosphorus retention forsamples (0.1 g) of the respective examples immersed in 100 cc of purewater at room temperature is shown in FIG. 8.

In FIG. 8, the amount of phosphorus retained was measured by an EDXanalyzer (model designation: EMAX240, manufactured by HORIBA Ltd.).

The results shown in FIG. 8 confirm that about 60% of phosphorus, about80% of phosphorus, and about 90% of phosphorus (i.e., proton conduction)remain in the samples from examples 1, 2, and 3, respectively. Thisdemonstrates that the catalyst-supporting carriers of the presentinvention maintain their proton conducting function even in a humidenvironment over a long period of time.

These catalyst-supporting carriers can be used for fuel cells, and areparticularly useful as the reaction layers of fuel cells. Within thereaction layer is where oxygen or hydrogen supplied from the outsidethrough the backing layers is ionized, and the reaction layer isnormally arranged between the proton exchange membrane and the backinglayer.

Examples of a method for producing a supported metal catalyst will nextbe described.

Each of the catalyst-supporting carriers produced above (Examples 1-3)is ground to powder by a ball mill or the like, and the resultingcatalyst-supporting carriers in particulate form are used to support aplatinum catalyst or other catalyst. The platinum catalyst may besupported on the catalyst-supporting carrier by any of the same methodsconventionally used to support platinum on a carbon carrier to form asupported platinum catalyst, including use of the colloid method orimpregnation.

The colloid method was performed as follows: A Pt colloidal solution wasprepared from platinic chloride. To this colloidal solution was addedthe catalyst-supporting carrier in the form of powder. The carrier wasthen subjected to a reduction treatment to obtain a carrier withplatinum supported thereon (Example 4). The supported platinum catalystof Example 5 was obtained by changing the platinum concentration.

The impregnation method was performed as follows: Thecatalyst-supporting carrier was ground and added to a methanol solutionof diamino platinum nitrite, which was then stirred and dried.Subsequently, the carrier was subjected to a reduction treatment toprovide the supported platinum catalyst of Example 6.

The characteristics of the supported platinum catalysts of theseexamples are as follows:

Example 4 Example 5 Example 6 Density of supported 30 50 30 platinum (wt%) Platinum particle size 5.1 5.1 3-6 (nm)

The foregoing densities of supported platinum were measured by an EDXanalyzer (model designation: EMAX240, manufactured by HORIBA Ltd.).

The foregoing particle sizes of platinum were measured by an XRD (modeldesignation: XPM3, manufactured by Bruker AXS Inc.).

Next, a fuel cell 20 as shown in FIG. 9 was constructed from thesupported platinum catalyst of Example 4. In particular, the supportedplatinum catalyst of Example 4 was dispersed in a mixture of pure water,a PTFE fluid dispersion, and alcohol, which was then applied onto abacking layer 26 made of carbon to provide a reaction layer (aircathode) 23. The amount of supported catalyst applied was varied toprepare three different reaction layers (Examples 7, 8, and 9). Thereaction layer (the anode or counter electrode) was made by applyingpowdered carbon with supported platinum (of a density of 40 wt %)dispersed in a mixture of pure water and a Nafion solution onto a carbonbacking layer 27. Subsequently, the backing layer 26 (coated with thereaction layer 23), the Nafion membrane 21, and the backing layer 27(coated with the reaction layer 24) were laminated and joined by hotpressing to obtain the fuel cell 20 of FIG. 9.

In the embodiment of fuel cell 20 shown in FIG. 9, no proton conductionwas included in the reaction layer on the air cathode side in order toprovide the fuel cell of Comparative Example 4. Thus, powdered carbonwith supported platinum (of a density of 40 wt %) was dispersed in amixture of a PTFE fluid dispersion and alcohol, which was then appliedonto a backing layer 26 made of carbon to provide the reaction layer(air cathode) of Comparative Example 4.

The fuel cells of Examples 7-9 and Comparative Example 4 were set in themeasuring apparatus 30 shown in FIG. 10 to measure the electrochemicalsurface areas of the cells. The following method of measurement wasused.

First, to humidify the sample sufficiently, humidified nitrogen gas wasintroduced to the reaction layer 23 on the air cathode side and to thereaction layer 24 on the anode side.

Second, the supply of nitrogen gas was terminated and replaced with ahydrogen gas supply to electrochemically stabilize the platinum surfacewhile applying 0.1-1.0 V, 50 mV/sec relative to the reference potentialof the reaction layer on the anode side.

CO gas was then introduced to the reaction layer on the air cathode sidewhile a potential of 0.3 V was maintained relative to the referencepotential of the reaction layer on the anode side so as to absorb CO inthe platinum. Next, while maintaining the potential, nitrogen gas wasintroduced to the reaction layer on the air cathode side to replace theCO in the reaction layer with nitrogen gas.

The CO absorbed in the platinum surface is electrochemically removed byoxidation. The electrochemical surface area of platinum was calculatedfrom the electrical quantity required for the removal by oxidation. Theequation below was used to calculate the electrochemical surface area.The test was conducted at 50° C.

Electrochemical surface area (cm²−Pt)=electrical quantity required foroxidizing CO (mC)/0.42 mC/cm²−Pt, wherein “mC is millicoulombs.

Table 1 shows the results of the test.

TABLE 1 Electrochemical surface areas Comparative Example 4 Example 7Example 8 Example 9 Pt surface area 274 133 174 210 (cm²-Pt)/(cm²-geometic) Pt electrochemical 17 139 163 179 surface area (cm²-Pt)/cm²-geometic) Efficiency of 6.3 104.5 93.7 85 Utilization of platinum(%)

In Table 1, the platinum surface area in each example was calculatedfrom the amount of platinum on the electrodes and the platinum surfacearea as obtained from the catalyst particle size which in turn wasmeasured by XRD. The electrochemical surface area was calculated fromthe quantity of electricity required for CO oxidization. The efficiencyof utilization of platinum was obtained by dividing the electrochemicalsurface area of platinum by the platinum surface area.

The results of Table 1 clearly show that the supported platinumcatalysts of the examples in accordance with the present invention havean electrochemical surface area of platinum even without an ion exchangeresin (such as Nafion).

The apparatus shown in FIG. 10 was used to measure the characteristicsof the fuel cell of Examples 7-9 and Comparative Example 4 at 50° C. byintroducing hydrogen gas to the reaction layer on the anode side and airto the reaction layer on the air cathode side, at atmospheric pressure.

FIG. 11 shows the results of the measurement. The fuel cells that usedthe supported platinum catalysts of Examples 7-9 as the reaction layersexhibited desirable current-voltage characteristics, confirming theirexcellence in operation in fuel cells.

In the tests described above, the supported catalysts in the examplesexhibited both proton conduction and electronic conduction at a lowtemperature, i.e. in a range of from room temperature (20° C.) to 60° C.While dependent on the presence of water, it is believed that thecatalyst-supporting catalysts exhibit equivalent functions andcharacteristics up to 200° C. in an unhumidified atmosphere.

The foregoing experiments (examples) demonstrate that the supportedplatinum catalysts exhibit their desirable functions in fuel cells evenin a low temperature range, in contrast to the conventional carriersupported, inorganic-based catalyst which exhibits its catalyticfunctions only at a high temperature of about 800° C.

Furthermore, as shown in FIG. 5, the electronic conductor phases (units)7 are connected to the proton conductor phases (units) 9 by covalentbonding, so that they are very close to each other. Because of this,even if a catalyst particle is very small, the electron conductors 7 andthe proton conductors 9 are both in contact with each catalyst particlesimultaneously. This makes it possible to supply electrons and protonsnecessary for a catalytic reaction to the catalyst in proper quantitiesand thereby improve catalyst utilization efficiency and thus fuel cellutilization efficiency.

Other embodiments of catalyst-supporting carriers that can be used forthe reaction layer of a fuel cell will be described hereinafter.

In accordance with Chemical Formula 2 above, resorcinol (5 g) wasdissolved in pure water (20 ml) and formaldehyde (6.7 ml) was addedthereto. Trimethyl phosphate (5.2 ml) was stirred in a mixed solution ofpure water (3.2 ml), ethanol (10.5 ml), and hydrochloric acid (124 μl)for one hour to hydrolyze the trimethyl phosphate. The solution obtainedby hydrolyzing trimethyl phosphate was added to theresorcinol/formaldehyde aqueous solution. Na₂CO₃(0.47 g) was then addedand the resultant solution was left standing at room temperature for 24hours to gelate.

The resultant gel was ground and was refluxed in a mixed solution oftrimethyl phosphate (5.2 ml), pure water (3.2 ml), ethanol (10.5 ml),and hydrochloric acid (124 μl) in an oil bath at a temperature of 200°C. for four hours. FIG. 12 shows the apparatus used for the refluxing.The supported catalyst of the embodiment was obtained by filtering,drying, and subjecting the resultant sample to heat-treatment in aninert atmosphere at 1000° C. for four hours.

The supported catalyst thus obtained was ground in a ball mill andformed into disk-shaped samples having a diameter of 15 mm and athickness of about 3 mm using an SPS sintering machine. Each sample wasinterposed between Nafion membranes as well as catalyst layers to form aholder shown in FIG. 6. The resultant holder was placed in a container,and nitrogen gas and hydrogen gas at a temperature of 60° C. and ahumidity of 100% were introduced into the container. A voltage wasapplied to the holder to obtain a response current. A voltage-currentcharacteristic of the supported catalyst of Example 10 was determinedfrom the response current (similar to that shown in FIG. 7). The protonconductivity was determined from this characteristic by calculation as5.6×10⁻³S/cm. The amount of phosphorus in the sample (Pmol/Cmol) was4.8% as determined by an EDX analyzer.

Another embodiment of a supported catalyst that can be used in thereaction layer of a fuel cell will now be described.

In accordance with Chemical Formula 3, resorcinol (2 g) and formaldehyde(2.7 ml) were dissolved in pure water (8 ml). Further, trimethylphosphate (4.2 ml) was stirred in a mixed solution of pure water (2.6ml), ethanol (5.0 ml), and hydrochloric acid (99 μl) for one hour tohydrolyze the trimethyl phosphate. The second solution was added to thefirst solution. Na₂CO₃ was then added, the solution was stirred at roomtemperature for three hours, then left standing at 60° C. for 24 hours,and then at 80° C. for 24 hours.

The sample was then heated in an autoclave having an internal space of120 cc (see FIG. 13) at 150° C. for six hours. The internal pressurewithin the autoclave rose to about 3-4 MPa due to the self-developedpressure of the solvents.

The supported catalyst of this embodiment was obtained by filtering,drying, and subjecting the resultant sample to heat-treatment in aninert atmosphere at 800° C. for four hours.

The supported catalyst thus obtained was ground in a ball mill andpressure-formed into disk-shaped samples having a diameter of 15 mm anda thickness of about 3 mm using an SPS (spark plasma sintering)sintering machine. Each sample was interposed between Nafion membranesalong with catalyst layers to form a holder as shown in FIG. 6. Theholder was placed in a container, and nitrogen gas and hydrogen gas at atemperature of 60° C. and a humidity of 100% were introduced into thecontainer. A voltage was applied to the holder to obtain a responsecurrent. A voltage-current characteristic of the supported catalyst ofExample 11 was determined from the response current (similar to thatshown in FIG. 7). The proton conductivity was determined from thischaracteristic by calculation as 1.5×10⁻²S/cm. The amount of phosphorusin the sample (Pmol/Cmol) was 5.8%. The phosphorus amount in thisexample was determined in the same manner as in Example 4.

Table 2 below and FIG. 14 show the relationship between the phosphoruscontent and the proton conductivity of each of the catalyst-supportingcarriers of the foregoing Examples.

TABLE 2 Example 2 Example 3 Example 10 Example 11 Pretreatment 200° C.autoclave of precursors Heating 800 1000 1000 800 temp (° C.) Phoshorus4.2% 3.8% 4.8% 5.8% content Proton 1.3*10⁻³ 7.3*10⁻⁴ 5.6*10⁻³ 1.5*10⁻²conductivity

These comparisons demonstrate that heating or pressure-heating ofprecursors gives an increased phosphorus content and thus improvedproton conductivity to the catalyst-supporting carrier obtained by theheat treatment.

Two grams of phenol were dissolved in a mixed solution of 5 cc ofethanol in 30 cc of pure water, to which 3.15 cc of a formaldehydesolution was then added. After 4.89 cc of a trimethyl phosphate solutionwas further added, the solution was stirred for one hour, and then 0.089gram of sodium carbonate was added. The solution was then stirred atroom temperature overnight. After the solution was left standing sealedat 70° C. for 24 hours, the solvent was removed. The resultant samplewas subjected to heat treatment at 500° C. in a nitrogen gas atmospherefor 4 hours to obtain the supported catalyst of this example.

Two grams of pyrogallol were dissolved in 8 cc of pure water, to which2.36 cc of a formaldehyde solution was then added. After 3.65 cc of atrimethyl phosphate solution was added, the solution was stirred for onehour, and then 0.0167 gram of sodium carbonate was added. The solutionwas then stirred at room temperature for three hours. After the solutionwas left at rest, sealed, at 50° C. for 24 hours, it was held sealed at80° C. for 72 hours. The resultant gel was subjected to heat treatmentat 800° C. in a nitrogen gas atmosphere for 4 hours to obtain thesupported catalyst of this example.

Three grams of dihydroxybiphenyl were dissolved in 12 cc of a mixedsolution of ethanol and water, with an ethanol/water volume ratio of1/1, to which 4.84 cc of a formaldehyde solution were then added. After7.49 cc of a trimethyl phosphate solution were added, the solution wasstirred for one hour, and then 0.0683 gram of sodium carbonate wasadded. The solution was then stirred at room temperature for threehours. After the solution was held sealed and at rest at 50° C. for 24hours, it was held sealed and at rest at 80° C. for an additional 72hours. Upon evaporation of the solvent, the resultant sample wassubjected to heat treatment at 500° C. in a nitrogen gas atmosphere for4 hours to obtain the supported catalyst of this example.

Three grams of resorcinol were dissolved in 12 cc of pure water, towhich 4.05 cc of a formaldehyde solution were then added. Whilestirring, 0.736 cc of an aqueous phosphate solution was gradually added.The solution was left sealed and standing at 70° C. for 24 hours, andthe solvent was then removed. The resultant sample was subjected to heattreatment at 1000° C. in a nitrogen gas atmosphere for 4 hours to obtainthe supported catalyst of this example.

Three grams of resorcinol were dissolved in 12 cc of pure water, towhich 4.05 cc of a formaldehyde solution were then added. Then, 0.028gram of sodium carbonate was added. After the solution was left standingsealed at 50° C. for 24 hours and then at 80° C. for 72 hours, the gelwas ground. The ground gel was separately washed with a 0.1Nhydrochloric acid solution, with pure water, and with ethanol in thatorder.

The washed gel was immersed in a solution of 1.5 gram oftungstophosphoric acid in 50 cc of ethanol. After being immersed at 50°C. for 48 hours, the gel was subjected to heat treatment at 700° C. in anitrogen gas atmosphere for 4 hours to obtain the supported catalyst ofthis example.

Three grams of resorcinol were dissolved in 12 cc of pure water, towhich 4.05 cc of a formaldehyde solution were then added. Added to thiswere a solution of 2.18 grams of phenylphosphonic acid in a mixture ofethanol and pure water, with an ethanol/pure water volume ratio of 1/1,and then 0.114 gram of sodium carbonate. After being stirred at roomtemperature for 12 hours, the solution was left standing at 60° C. for24 hours and then at 80° C. for an additional 48 hours both in a sealedcondition. The resultant sample gel was subjected to heat treatment at800° C. in a nitrogen gas atmosphere for 4 hours to obtain the supportedcatalyst of this example.

Samples obtained in examples 12-17 were ground in a ball mill and werepressure-formed into disks having a diameter of 15 mm and a thickness ofabout 3 mm. To measure the electronic specific resistance of eachsample, the sample was interposed between collector plates made of goldand a DC current was applied to produce a response voltage. Theelectronic specific resistance was obtained from the response voltage.To measure the ionic conductance of the obtained samples, each samplewas interposed between Nafion membranes along with catalyst layers toform a holder as shown in FIG. 6. The holder was placed in a container,and nitrogen gas or hydrogen gas at a temperature of 60° C. and ahumidity of 100% was introduced into the container. A voltage wasapplied to the holder. The ionic conductance was obtained from theresponse current.

The results are shown in Table 3.

TABLE 3 Proton Electronic specific conductance resistance (Ω · cm)(S/cm) Example 12 10 7.1 × 10⁻⁴ Example 13 0.18 1.1 × 10⁻² Example 14 505.3 × 10⁻⁴ Example 15 0.07 1.0 × 10⁻³ Example 16 0.14 1.5 × 10⁻⁴ Example17 0.14 2.7 × 10⁻³

As in Examples 1-3, the stability of these Examples exposed to water wasconfirmed by immersing 0.1 gram of each sample in pure water at roomtemperature and measuring the concentration of phosphorus in theimmersed sample (the tungsten concentration in Example 12) over time.The concentrations of phosphorus (the tungsten concentration in Example12) substantially stabilized after 50 hours of immersion. Even after 200hours of immersion, with respect to the initial phosphorus concentration(the tungsten concentration in Example 12), 45%, 81%, 86%, 90%, 95%, 75%of phosphorus (or tungsten) remained in the samples in Examples 12, 13,14, 15, 16, and 17, respectively.

In has now been discovered by the present inventor that a novel protonexchange membrane can be used to advantage as an element of a fuel cellas shown in FIG. 9, instead of Nafion (polymer proton exchangemembrane). The new proton exchange membrane makes it possible to operatethe fuel cell, especially the reaction layer, at 100 degrees C. or more,with effective use of the exhaust heat therefrom. The novel protonexchange membrane may be a membrane formed of an organic-inorganichybridized compound including silicon alcoxide at the end thereof, aninorganic-organic hybridized material including a phosphate group, agrass system solid electrolyte such as P₂O₅—Mo_(x), a proton exchangemembrane made by impregnating phosphoric acid into a porous medium suchas silicon carbide or a basic polymeric membrane.

The reaction layer is made in a same manner as shown in FIG. 9. In thismanner, both the cathode and anode can be made without using an ionexchange membrane.

To provide an inorganic-organic hybridized membrane including phosphategroups, polyethylene glycol and 3-isociarate propyl triethoxysilane arereacted to form an inorganic-organic hybridized compound, and furtherreacted with a phosphate compound as a source of phosphoric acid.

The reaction layer and the backing layer are formed in the manner shownin FIG. 9.

The fuel cell can be operated at temperatures within a range from 100degrees C. to 200 degrees C.

Alternatively, as the novel proton exchange membrane, a fuel cell mayhave a glass system solid proton exchange membrane. A metal oxide systemsolid proton exchange membrane (P2O5-Mox (M=Si, Ti, Zr, Al etc.) made bythe sol-gel method is a glass system solid proton exchange membrane. Toshape the glass system solid proton exchange membrane, the moldingmaterial in a sol state is molded or cast, followed by drying. Thereaction layer and the backing layer are formed in the same manner asshown in FIG. 9. This fuel cell can also be operated at a temperaturewithin a range from 100 degrees C. to 200 degrees C.

FIG. 15 shows a phosphoric-acid fuel cell system including, as areactive layer, a supported metal catalyst. The phosphoric-acid fuelcell system 110 includes a fuel cell 112, separators 114 a, 114 b andpower collecting electrodes 116 a, 116 b. The fuel cell 112 comprises aproton exchange membrane, reactions layers and backing layers. Theseparators 114 a, 112 b are disposed on opposing sides of the fuel cell112 and are bipolar plates. The power collecting electrodes 116 a, 116 bare disposed on outer sides of the separators 114 a and 114 b,respectively. End plates 118 a and 118 b are disposed on outer sides ofthe power collecting electrodes, respectively, and the end plates 118 aand 118 b are tightened together by bolts (not shown). The fuel cell 112comprises an electrolyte matrix layer 120 of porous silicon carbidecontaining phosphoric acid groups, and the reaction-backing layers 124,126 are disposed on each side of the electrolyte matrix layer 120 byinterposing a frame-like spacer 122. The supported platinum catalyst ofthe Example 4 is dispersed in a mixed liquid comprising pure water, aPTFE liquid dispersion and alcohol and then coated onto a carbon backinglayer to produce the reaction-backing layers 124 and 126. A first gaspath 128 for flow of hydrogen gas is formed in the separator 114 a onthe surface facing the reaction-backing layer 124. Likewise, a secondgas path 130 for flow of air is formed in the separator 114 b on thesurface facing the reaction-backing layer 126.

The phosphoric-acid fuel cell thus constructed can be operated with itsreaction layer at a temperature within a range from 100 degrees C. to200 degrees C.

The present invention is not limited at all by the embodiments and thedescription of the embodiments. The present invention also containsvarious changes and modifications thereto without departure from thedescription of claims which follow in a range that can be easilyattained by a person having ordinary skill in the art.

1. A fuel cell including a reaction layer comprising a catalyst carrierformed of a compound containing inorganic electron conductor units andinorganic proton conductor units together formed in its molecularstructure and a catalyst supported on the catalyst carrier.
 2. The fuelcell according to claim 1, wherein said electron conductor units areobtained by carbonizing at least one carbon source selected from thegroup consisting of aliphatic hydrocarbons, aromatic hydrocarbons andderivatives thereof.
 3. The fuel cell according to claim 2, wherein saidcarbon source is at least one member selected from the group consistingof polyacetylene, resorcinol, phenol, phenylphenol, polyaniline,polypyrrole, polythiophene, phenylphosphonic acid, phenylsilanealkoxide, pyrogallol, and dihydroxybiphenyl.
 4. The fuel cell accordingto claim 1, wherein said electron conductor units are formed from acarbonaceous material.
 5. The fuel cell according to claim 1, whereinsaid proton conductor units are formed from at least one member selectedfrom the group consisting of phosphorus-containing compounds,sulfur-containing compounds, carboxylic acids, boric acid, and inorganicsolid-state acids.
 6. The fuel cell according to claim 1, wherein saidelectron conductor units have consecutive carbon-carbon bonds includingcarbon-carbon double bonds.
 7. The fuel cell according to claim 1,wherein said catalyst is a noble metal.
 8. A fuel cell having a reactionlayer containing a catalyst support comprising electron conductor unitsmade of an inorganic material obtained by carbonizing an organicmaterial fixed to proton conductor units made of an inorganic material.9. The fuel cell according to claim 8, wherein the electron conductorunits are fixed to the proton conductor units by covalent bonding. 10.The fuel cell according to claim 8, wherein the proton conductor unitsare fixed to the electron conductor units by intercalation.
 11. The fuelcell according to claim 8, wherein the proton conductor units are fixedto the electron conductor units by inclusion.
 12. The fuel cellaccording to claim 5, wherein the reactive layer operates at atemperature within a range from about 100 degree C. to about 200 degreeC.
 13. A method for producing a reaction layer wherein the reactionlayer comprises a catalyst carrier formed of a compound containinginorganic electron conductor units and inorganic proton conductor unitstogether formed in its molecular structure and a catalyst supported onthe catalyst carrier for a fuel cell comprising: forming a highmolecular weight precursor by mixing and polymerizing at least onecarbon source selected from the group consisting of aliphatichydrocarbons, aromatic hydrocarbons and derivatives thereof, with aproton conducting material; pyrolizing the high molecular precursor toobtain an inorganic polymer; supporting a catalyst on the inorganicpolymer to form a supported catalyst; and joining the supported catalystto a proton exchange layer.
 14. The method according to claim 13,wherein said carbon source is at least one member selected from thegroup consisting of polyacetylene, resorcinol, phenol, phenylphenol,polyaniline, polypyrrole, polythiophene, phenylphosphonic acid,phenylsilane alkoxide, pyrogallol, and dihydroxybiphenyl.
 15. The methodaccording to claim 13, wherein said proton conducting material containsat least one member selected from a group consisting ofphosphorus-containing compounds, sulfur-containing compounds, carboxylicacids, boric acid, and inorganic solid-state acids.
 16. A method forproducing a reaction layer wherein the reaction layer comprises acatalyst carrier formed of a compound containing inorganic electronconductor units and inorganic proton conductor units together formed inits molecular structure and a catalyst supported on the catalyst carrierfor a fuel cell comprising: forming a high molecular weight precursor bypolymerizing at least one carbon source selected from the groupconsisting of aliphatic hydrocarbons, aromatic hydrocarbons andderivatives thereof to form a polymer; and introducing a protonconducting material into said precursor; pyrolizing the precursor toform an inorganic polymer; supporting a catalyst on the inorganicpolymer to form a supported catalyst; and joining the supported catalystto a proton exchange layer.
 17. The method according to claim 16,wherein said carbon source is at least one member selected from thegroup consisting of polyacetylene, resorcinol, phenol, phenylphenol,polyaniline, polypyrrole, polythiophene, phenylphosphonic acid,phenylsilane alkoxide, pyrogallol, and dihydroxybiphenyl.
 18. The methodaccording to claim 16, wherein said proton conducting material containsat least one member selected from the group consisting ofphosphorus-containing compounds, sulfur-containing compounds, carboxylicacids, boric acid, and inorganic solid-state acids.
 19. A method ofproducing a reaction layer wherein the reaction layer comprises acatalyst carrier formed of a compound containing inorganic electronconductor units and inorganic proton conductor units together formed inits molecular structure and a catalyst supported on the catalyst carrierfor a fuel cell comprising: binding or mixing an organic compound with acompound having mobile protons to obtain a high polymer weightprecursor; carbonizing said high polymer weight precursor to form aninorganic polymer as a catalyst carrier; supporting a catalyst on thecatalyst carrier to form a supported catalyst; and joining the supportedcatalyst to a proton exchange layer.